November 2018, ScientificAmerican.com 35
Methane
2CH 4
ANME
8 electrons
Sulfate
2(SO 4 )2-
SRB
åù ̈my
Bicarbonate 2HS-
2HCO 3 - Waterer
2 2HH 2 OO
WWatWatteer
HH 22 OO
CaCaCalciucium
Ca2+
Calcium
carbonate
CaCO 3
Cararbobonbonb
mm¹¹¹āmāmmyyy
COOO 22
3ù ̈my
2H 2 S
ĂmĂmà¹āà¹my
2 2OH2OH2OH--
'āĂ'āĂ'āĂ'āĂyy ́y ́y ́
4 4O 22
Ămà¹y ́¹ ́ ́ ́ ́åååå
4H4H+
Sulfate
2(SO 4 )2-
ANME/SRB clumps
Methane
CH 4
3ym®y ́ï
Microbial mat
Rock
Noot tosscccaleaa
Illustration by Tami Tolpa
SOURCES: “A MARINE MICROBIAL CONSORTIUM APPARENTLY MEDIATI
NG ANAEROBIC OXIDATION OF METHANE,” BY ANTJE BOETIUS ET AL., IN
NATURE,
VOL. 407; OCTOBER 5, 2000; “SINGLE CELL ACTIVITY REVEALS DIRECT ELECT
RON TRANSFER IN METHANOTROPHIC
CONSORTIA,” BY SHAWN E. MCGLYNN ET AL., IN
NATURE,
VOL. 526; OCTOBER 22, 2015; “METHANE-DERIVED CARBONATES AND AUTH
IGENIC PYRITE FROM THE NORTHWESTERN BLACK SEA,” BY J. PECKMANN ET
AL., IN
MARINE GEOLOGY,
VOL. 177, NOS. 1–2; JUNE 30, 2001
these species, as well as how cells and biomolecules work in
general. When researchers then zoomed out from these studies
to understand the biosphere as a whole, stitched together from
the constituent species, however, major gaps in their knowledge
remained. Only a tiny fraction of microbes seen in the wild could
be isolated, suggesting that the species making up complex natu-
ral communities are intertwined in ways that cannot easily be
replicated in the laboratory. And the very coexistence of many
thriving species, which often play complementary roles, seemed
to contradict the conventional wisdom that microbial eco-
systems revolve around a winner-take-all struggle for resources.
Moreover, metabolic activity rates of individual species
measured in the lab—such as how quickly they produce oxygen
or consume nitrogen—rarely matched values from real-world
environments because species that can be isolated in the lab are
often more vigorous than those that cannot be. In other words,
the whole was sometimes more, sometimes less, but always
different from the sum of the parts.
But a growing body of evidence suggests that these discon-
nects can be reconciled by considering the vital importance of
interactions among organisms. Over the past decade advances in
bio molecular sequencing and microscopic imaging, among other
technologies, have en abled researchers to study microbial com-
munities more holistically than ever before. The latest findings
indicate that collaboration is a critical driver of the biosphere: as
individual organisms evolve to share energy, genetic information
and metabolic duties, they unlock new ways of life and gain entry
to previously inaccessible habitats.
The Mystery of the
Missing Molecules
In layers of sediment and rock under the sea-
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A Classic Symbiosis
Methane molecules are rich in energy, but their
chemical stability makes it tough to access their
energy. Anaerobic methanotrophs (ANME) can
crack these molecules open, but the electrons
that are released ● 1 can accumulate, slowing
metabolism. Luckily for the methanotrophs,
nearby sulfate-reducing bacteria (SRB) take up
the excess electrons ● 2 , using them to convert
åù ̈Dïy ́ï¹åù ̈my● 3 and harvesting the resulting
energy. Ultimately much of the methane-derived
carbon precipitates as calcium carbonate rock ● 4 ,
building large mounds at methane seeps.
1 2
3
4